27 Every one is justified in using as much force as is reasonably necessary
(a) to prevent the commission of an offence
(i) for which, if it were committed, the person who committed it might be arrested without warrant, and
(ii) that would be likely to cause immediate and serious injury to the person or property of anyone; or
(b) to prevent anything being done that, on reasonable grounds, he believes would, if it were done, be an offence mentioned in paragraph (a).